A rare series of two atmospheric rivers is poised to deliver significant rainfall to Washington, marking an unusual event for this time of year. Typically, atmospheric rivers, which are narrow corridors of concentrated moisture in the atmosphere, are more common during the fall and winter months. However, this late-season setup will bring substantial precipitation and the potential for flooding to the region.

As of 9 AM Sunday morning, the first of two atmospheric rivers has made landfall in Washington. The second atmospheric river is expected to arrive early Tuesday morning, resulting in additional rainfall throughout the day. In the precipitable water map below, you can see the two atmospheric rivers moving into Washington first on Sunday and then once again on Tuesday. 

Atmospheric rivers along the West Coast are rated on a scale from 1 to 5, which characterizes the strength and impacts of atmospheric rivers based on their water vapor content and duration, as described below.

• AR5: Exceptional
• AR4: Extreme
• AR3: Strong 
• AR2: Moderate
• AR1: Weak

The first atmospheric river is forecast to reach category 3 Washington. 

Image: Atmospheric river scale forecast for northern California through British Columbia. Courtesy of CWWE.

The two atmospheric rivers are shown below IVT maps below. These maps illustrate the movement and intensity of moisture in the atmosphere. They show the flow of water vapor transported by winds over a specific area.

Image: Plume of atmospheric river moisture stretching across the North Pacific and impacting Washington on Sunday. Courtesy of CWWE.

Image: Plume of moisture from the second atmospheric river stretching across the North Pacific and impacting Washington on Tuesday. Courtesy of CWWE.

Rain

As the moisture plume from the first atmospheric river moves inland today, heavy rain will overspread the western part of the state. Moderate to heavy rain will continue throughout the day on Sunday, and persist through Monday afternoon. More rain is expected Monday night through Tuesday, albeit at a lesser intensity.

By the time the second of the two atmospheric rivers weakens and drops south of Washington on Tuesday, coastal areas could see 2 - 4 in of rain. In the Olympic and Cascade mountains, 3 - 5 in is possible. Urban areas like Seattle are also expected to receive heavy rainfall, with 1 - 2 in possible by Tuesday. The latest rainfall forecast from the National Blend of Models is shown below.

Flooding

Despite the antecedent drought conditions, flooding is still possible along and west of the Cascades. Ahead of this storm, the U.S. Drought Monitor had more than 25 percent of the state under moderate drought or worse. 

Image: Current drought severity across Washington, with a large area of D1 Moderate Drought conditions.

Combined with the high snow levels of the storm, this deluge will result in excessive runoff. This may lead to flooding of rivers, creeks, streams, and other low-lying, flood-prone areas. Flooding is possible in poor drainage and urban areas, while some creeks and streams may rise over their banks. During this time of year, mountain recreation activity is increasing. Small streams and rivers will be unusually high and very cold, so caution must be taken. 

The latest flood and hydrological watches and warnings from NWS Seattle are shown below. 

An unusual setup with back-to-back atmospheric rivers is set to impact Washington starting on Sunday, bringing heavy rainfall and potential flooding through Tuesday. This stormy period will deliver heavy rain across the state, especially in the Cascade and Olympic mountains. The heaviest rainfall is expected on Sunday, and concerns about urban and small-stream flooding will be rising. Residents are advised to stay informed and take precautions as this system progresses​ through the early part of the work week. 

Luke Stone
Forecaster, OpenSnow